Honeycomb structures such as honeycomb sandwich panels are widely known and are used in various different fields such as, for example, commercial and military aircraft, marine vessels, automobiles, buildings and construction, sporting equipment and even toys.
A typical prior art honeycomb sandwich panel consists of two relatively stiff and thin face sheets made from a dense material such as metal or fibre composite which is adhesively or otherwise bonded to a relatively thick and light honeycomb core. FIG. 1 illustrates schematically a pair of such face sheets 1, 2 and a honeycomb core 3. As is conventional, the honeycomb core is made up from a network of thin walls 4, which in the illustrated arrangement is provided in the form of a square grid arrangement. The network of walls thus defines a plurality of discrete hollow cells 5 between individual walls 4. The cells 5 may be columnar. As will be recognised by those of skill in the art, other configurations for the network of walls 4, and thus the resulting cells 5, are also possible, and indeed configurations having hexagonally shaped cells 5 are widely used; such arrangements clearly being most directly suited to the general term “honeycomb structure” on account of sharing their shape and configuration with natural honeycombs from which the term derives. FIG. 1 illustrates the two face sheets 1, 2 being offered up to respective opposite sides of the honeycomb core 3 for bonding to the core. FIG. 2 illustrates the completed structure.
Honeycomb panels are commonly used in situations where saving weight in a structure is important. They offer considerably higher strength-to-weight and stiffness-to-weight ratios than alternative solid panels of similar weight, size and geometry. Additionally, honeycomb panels are well suited to use in acoustic damping and thermal insulation.
It is for these reasons that acoustic liners for the engine casings of modern gas turbine engines in the aeronautical industry are fabricated from honeycomb panels of the general type described above. In particular, such acoustic liners are often fabricated using a fibre (such as glass, nylon, etc.) reinforced plastic or aluminium perforate front face sheet bonded to one side of an aluminium honeycomb core, with a glass reinforced plastic rear sheet bonded to the opposite side of the core. However problems can arise with such panels which are attributable to their basic construction.
It has been found that currently used designs of honeycomb panels have their strength and durability limited by the strength of the bond, which as explained above is usually created by the use of adhesive, which exists between each face sheet and the honeycomb core. This is because the strength of the bond between each face sheet and the core is determined by the total contact area over which the adhesive can effectively bond the face sheet and the core together.
As will be appreciated from FIG. 1, the actual contact area presented by the network of thin walls 4 is very small in comparison to the overall area of the core 3 and the face sheets 1, 2; simply due to the thin configuration of the walls 4. Additionally, the bond between the core 3 and each face sheet 1, 2 provided by adhesive applied to the ends of the thin walls 4 will be subject to peel and tensile forces in the use of many panels fabricated in the conventional manner.
Although it is common to apply the adhesive in the form of a fillet, the total area available for the fillet is often small and is dependent on the ability of the adhesive to form a meniscus along the cell walls due to capillary action and/or the tendency of some parts of the cell wall to sink into the film or paste adhesive used for the bonding. However, it is worthy to note that if thicker fillets of adhesive are used in an attempt to counteract this problem, there can often be a tendency for the adhesive to completely encapsulate the honeycomb structure of the core 3, which reduces the weight saving advantage of using a honeycomb structure.
Fabricating acoustic liner panels in the above-described prior art manner is very expensive and time consuming because of the care which must be taken when applying the adhesive to prevent it plugging the perforations which are usually provided in the front face sheets for such panels, which of course would reduce the panel's noise damping effect. To avoid this problem, the adhesive must be applied to the face sheet, or to the ends of the walls 4 of the core in a carefully reticulated manner, which is not only time consuming but also results in a reduction in the total contact area of the bond.
In addition, the adhesive bond formed between the honeycomb and the mating face sheets in the prior art panels are readily exposed to the risk of chemical hydrolysis of the adhesive and or corrosion of the bonding interface due to ingress and entrapment of water, lubricating oils or moisture around the bond. The resultant effect of either or both of these phenomena is a significantly lowered bond strength and service life of the honeycomb structure.
There is therefore a need for a honeycomb structure which is less susceptible to the above-mentioned problems and which, in particular is stronger and more durable than previous structures.